On Wed, Nov 13, 2024 at 04:08:39AM +0900, Vincent Mailhol wrote:
> __builtin_constant_p() is known for not always being able to produce
> constant expression [1] which lead to the introduction of
> __is_constexpr() [2]. Because of its dependency on
> __builtin_constant_p(), statically_true() suffers from the same
> issues.
>
> For example:
>
> void foo(int a)
> {
> /* fail on GCC */
> BUILD_BUG_ON_ZERO(statically_true(a));
>
> /* fail both clang and GCC */
> static char arr[statically_true(a) ? 1 : 2];
> }
>
> For the same reasons why __is_constexpr() was created to cover
> __builtin_constant_p() edge cases, __is_constexpr() can be used to
> resolve statically_true() limitations.
>
> Note that, somehow, GCC is not always able to fold this:
>
> __is_constexpr(x) && (x)
>
> It is OK in BUILD_BUG_ON_ZERO() but not in array declarations or in
> static_assert():
>
> void bar(int a)
> {
> /* success */
> BUILD_BUG_ON_ZERO(__is_constexpr(a) && (a));
>
> /* fail on GCC */
> static char arr[__is_constexpr(a) && (a) ? 1 : 2];
>
> /* fail on GCC */
> static_assert(__is_constexpr(a) && (a));
> }
>
> Encapsulating the expression in a __builtin_choose_expr() switch
> resolves all these failed test.
>
> Declare a new _statically_true() macro which, by making use of the
> __builtin_choose_expr() and __is_constexpr(x) combo, always produces a
> constant expression.
So, maybe name it const_true() then?
> It should be noted that statically_true() still produces better
> folding:
>
> statically_true(!(var * 8 % 8))
>
> always evaluates to true even if var is unknown, whereas
>
> _statically_true(!(var * 8 % 8))
>
> fails to fold the expression and return false.
>
> For this reason, usage of _statically_true() be should the exception.
> Reflect in the documentation that _statically_true() is less powerful
> and that statically_true() is the overall preferred solution.
>
> [1] __builtin_constant_p cannot resolve to const when optimizing
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
>
> [2] commit 3c8ba0d61d04 ("kernel.h: Retain constant expression output for max()/min()")
>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
> ---
> Bonuses:
>
> - above examples, and a bit more:
>
> https://godbolt.org/z/zzqM1ajPj
>
> - a proof that statically_true() does better constant folding than _statically_true()
>
> https://godbolt.org/z/vK6KK4hMG
> ---
> include/linux/compiler.h | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 4d4e23b6e3e7..c76db8b50202 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -308,6 +308,20 @@ static inline void *offset_to_ptr(const int *off)
> */
> #define statically_true(x) (__builtin_constant_p(x) && (x))
>
> +/*
> + * Similar to statically_true() but produces a constant expression
> + *
> + * To be used in conjunction with macros, such as BUILD_BUG_ON_ZERO(),
> + * which require their input to be a constant expression and for which
> + * statically_true() would otherwise fail.
> + *
> + * This is a tradeoff: _statically_true() is less efficient at
> + * constant folding and will fail to optimize any expressions in which
> + * at least one of the subcomponent is not constant. For the general
> + * case, statically_true() is better.
I agree with Rasmus. Would be nice to have examples where should I use
one vs another right here in the comment.
> + */
> +#define _statically_true(x) __builtin_choose_expr(__is_constexpr(x), x, false)
> +
> /*
> * This is needed in functions which generate the stack canary, see
> * arch/x86/kernel/smpboot.c::start_secondary() for an example.
> --
> 2.45.2
